Sodium cyanide (NaCN) has a variety of uses. For example, it is used in electroplating, treating metal surfaces, extracting and recovering metals from ores, and chemical uses.
Sodium cyanide (NaCN) for these uses is known to be produced by the so-called wet process or the neutralization of hydrogen cyanide (HCN) with sodium hydroxide (NaOH). The HCN is added both in the form of a gas or liquid, and the NaOH is added as an aqueous solution to form an aqueous NaCN solution. Solid NaCN crystals are formed during evaporation of the aqueous NaCN solution. These crystals can be separated and dried to produce an anhydrous NaCN product, which is generally compacted into briquettes for ease in shipment and handling.
Most often, producers use substantially pure anhydrous HCN to react with substantially pure NaOH generally fed as a 50% solution to the reactor. U.S. Pat. No. 2,708,151 to McMinn, Jr. and U.S. Pat. No. 2,726,139 to Oliver teach processes that use substantially pure HCN.
HCN is produced commercially by various processes well known in the art. With several of the known processes (for example, the Andrussow process, described in U.S. Pat. Nos. 1,934,838 and 1,957,749, which catalytically reacts methane, ammonia and air), the synthesis product is a mixture of components, including the desired HCN as well as water, unreacted ammonia, hydrogen, nitrogen and oxides of carbon. Where substantially pure HCN is required, complicated and expensive rectification and isolation procedures are necessary to provide a satisfactory product.
Since there would be considerable savings in investment and operating cost if the rectification and isolation procedures needed to purify HCN could be eliminated, there have been numerous attempts to use impure HCN gas to produce an aqueous cyanide solution susceptible to conversion to anhydrous NaCN by evaporative crystallization. When HCN synthesis gas is directly absorbed in NaOH, the aqueous solutions produced contain measurable quantities of impurities absorbed from the impure gases.
One of the primary impurities in the aqueous solution is sodium carbonate formed by reaction of carbon dioxide with the NaOH neutralizing agent. Sodium carbonate so formed is soluble in the saturated NaCN solution formed to about 1.5% by weight. During evaporation and crystallization of NaCN, the sodium carbonate will crystallize and become an impurity in the anhydrous NaCN product. In addition, since sodium carbonate has an inverse solubility relationship in aqueous NaCN solutions, less will stay in solution as the temperature of the solution is increased. Thus, it would be expected that sodium carbonate would precipitate and tend to plug heat exchangers where surface temperatures can be high, for example, the evaporator calandria heating surface would be expected to foul. As that heating surface begins to foul, heat transfer would be made more difficult thereby increasing the cost of operation. As more fouling occurs, one would expect eventual interruption of operation of the evaporative crystallizer.
U.S. Pat. No. 3,619,132 to Mann et al. uses impure HCN gas, but avoids sodium carbonate problems by absorbing impure HCN gas that is free of carbon dioxide in alkali hydroxide. Mann et al. use spacially separated steps of absorbing at subatmospheric pressure and crystallizing at still lower pressure.
Others have attempted to employ HCN containing carbon dioxide as an impurity, but remove the sodium carbonate prior to crystallization.
U.S. Pat. No. 2,616,782 to Cain teaches a process in which an oxide of calcium is added to the NaOH in an amount at least equivalent to the carbon dioxide in the HCN gas and the temperature is controlled at less than 196.degree. F. (about 91.degree. C.). The process is to reduce contamination due to sodium carbonate which forms by reaction of the carbon dioxide with the NaOH. Calcium carbonate, which is formed instead of the sodium carbonate, is insoluble is the NaCN solution and can be removed by filtration prior to crystallization.
U.S. Pat. No. 1,531,123 to Mittasch et al. also teaches a process for using HCN gas containing carbon dioxide. The process employs concentrated NaOH, low temperature (preferably less than 40.degree. C.) and adds ammonia to precipitate the sodium carbonate formed prior to crystallization.
In addition to the problems associated with carbon dioxide being present, water in the HCN synthesis gas presents other difficulties. The synthesis gases normally contain substantial quantities of water, most of which is condensed in the absorber at normally low absorption temperatures. The condensed water adds to the water load (water of reaction plus water from aqueous NaOH) that the crystallizer must handle. When the excess water is evaporated in the crystallizer, more water vapor must be vented. Increased venting of water tends to strip additional HCN vapors from the solution. The result is a disturbance of the equilibrium of the neutralization reaction. NaCN then reacts with water to form HCN and NaOH to bring the reaction back to equilibrium. This leads to lost yield, additional scrubbing requirements to remove the HCN in the vapors, and an increase of NaOH content in the evaporator (or crystal mother liquor). When levels of NaOH in the crystal mother liquor are high, the dry NaCN crystals become coated with NaOH. Since the NaOH is more hygroscopic than the NaCN, storage and handling of the anhydrous product becomes more difficult. Exclusion of atmospheric air from storage and shipping containers to avoid water absorption becomes even more critical and water absorption, when it does occur, leads to caking, for example.
Typically, the NaCN is formed into briquettes by dry compression methods and shipped to users who generally dissolve the NaCN in water to make an aqueous solution to be used in their process. For example, for use in the extraction of metals from ores, the solid NaCN product is made into a dilute solution containing about 23 weight percent NaCN. Extractors generally also add a base to raise the pH to minimize cyanide ionization thereby reducing evolution of cyanide vapors. To be acceptable, crystals must have a high enough NaCN concentration such that, when diluted, the weight percent NaCN is high enough for the intended purpose. For example, an assay of say 90 to 95% NaCN would be acceptable for metal extraction so long as the impurities do not interfere with crystal properties, particularly those that effect the storage and shipping ability and the effectiveness of the anhydrous NaCN in performing its intended purpose.